One known method for producing phosphorous pentoxide (P2O5, usually present as the dimer P4O10 in the gas phase) involves processing raw material agglomerates containing phosphate ore, silica, and coke in the bed of a rotary kiln to chemically reduce the phosphate ore and generate gaseous phosphorus metal (P4) and carbon monoxide (CO) off gas to the kiln freeboard where they are burned (oxidized) with air to provide heat for the process. It may be referred to as the kiln phosphoric acid (KPA) process. The oxidized phosphorus metal is a phosphorus oxide (normally, P4O10) which can be scrubbed from the kiln off gases with a phosphoric acid (H3PO4) solution and water to make a suitable phosphoric acid product.
KPA process chemistry is similar to another process known as the furnace acid process for manufacture of phosphoric acid. In the furnace process, the raw materials are heated and partially melted. An endothermic reduction reaction is carried out in one vessel called the electric furnace where the heat is supplied by the use of electric resistance heating in the bed. The phosphorus metal is recovered from the off gas of the furnace with cold water sprays as liquid phosphorus metal which can be transported to another vessel called the burner where it generates considerable heat while being burned with air. The resulting phosphorus oxide is absorbed in water to make a concentrated, high purity phosphoric acid.
The electric furnace in the furnace process does not use the heat generated from burning phosphorus metal that arises in the burner vessel. Also, the electric furnace does not use heat from burning the carbon monoxide that it generates. Although widely used in the last century for producing phosphoric acid, the cost of electricity as compared to the cost of sulfuric acid resulted in shutdown of most of the furnace acid plants in favor of another process known as the sulfuric acid process for making phosphoric acid.
If the heat generated in burning the off gasses from the furnace process reduction reaction could be utilized to provide the heat requirements of the reduction process, thereby replacing electrical heating, then economies might be realized. Converting the furnace process carbon reductant to carbon dioxide might generate sufficient heat, if used efficiently, to replace all the heat added by electricity in the furnace process. A vision of such potential motivated many researchers over the years to develop concepts where heat integration could be realized. The following references describe the various attempts: Levermore (U.S. Pat. No. 2,075,212), Lapple (U.S. Pat. Nos. 3,235,330 and 3,241,917), Saeman (U.S. Pat. No. 3,558,114), Megy (U.S. Pat. Nos. 4,351,809 and 4,351,813), Hard (U.S. Pat. No. 4,389,384), and Park (U.S. Pat. No. 4,420,466). All of the described processes use a rotary kiln with a reducing bed and an oxidizing freeboard and are collectively within the realm of KPA processes.
A kiln within a kiln concept forwarded in the Levermore patent addressed the heat integration issue in a reasonable conceptual way, but was not practical because no material of construction for the inner kiln was available. The Levermore process heated up the agglomerated solids containing phosphate ore, silica, and carbonaceous material and conducted the endothermic reduction reaction in an inner kiln held inside an outer kiln. The P4 and CO off gases from the reduction reaction in the inner kiln passed between the outside wall of the inner kiln and the inner wall of the outer kiln, where air was admitted to oxidize the P4 and CO, generating sufficient heat to supply the requirements of the inner kiln. The heat then passed through the outside wall of the inner kiln.
The Lapple and Megy patents recognized that a separate oxidizing freeboard and reducing bed could be maintained in a kiln without a separating wall, but failed because of melting problems during the phosphate reduction reaction. Lapple and Megy specify a calcium-to-silica mole ratio in the feed to the kiln greater than 1.0.
The Park patent describes a process requiring ore with less silica than is cheaply available and a very hot kiln operation that has not appeared attractive enough to encourage commercialization to date.
The process in the Saeman patent involves carrying out the rotary kiln reduction reaction in a molten slurry, contained within the shell of the kiln protected by freezing a layer of solids on the inside of the kiln wall. This process has been abandoned.
The process in the Hard patent showed promise and a continuous pilot plant based on a 33 inch diameter by 30 feet long kiln was operated under the direction of the present inventor in the early 1980's. The results were published in Leder, et al., A New Process for Technical Grade Phosphoric Acid, Ind. Eng. Chem. Process Des. Dev., 1985, 24, 888-897, but the process was abandoned as his teachings were not complete enough to show how to carry out an economic commercial process. The pilot plant yields were low, with a maximum yield of 72% when run in a commercial mode without bedding coke, and 86% when a large amount of bedding coke was used. Other problems included: 1) throughput rates that were low which indicated a high capital cost requirement for the process, 2) high temperature of operation (greater than 1435° C.) to reach high yields, which put the operation close to melting problems and required higher silica admixtures in the kiln feed than desired for commercial operation, and 3) high maintenance problems. The high temperature and partial oxidation of carbon from the kiln solids resulted in transfer of significant amounts of fluorine, sodium, potassium, and sulfur to the kiln off gas, producing deposits in the back end of the kiln and off gas lines, contamination of the product acid, and extra costs associated in scrubbing acidic gases from the process. The off gases from the process were reducing requiring the added cost of an after burner. The combination of these problems was such that no one has attempted to commercialize the Hard process even though it has now been over twenty years since the pilot plant was operated.
As may be appreciated from the previous difficulties described above, the KPA process may be improved further.